An important need of unmanned aerial vehicles (UAVs) exists for the military to perform aerial surveillance over ocean areas beyond line-of-sight from ships not large enough or suitable for manned surveillance flights, or from larger ships because the surveillance task is too hazardous for manned operations. Simple operation from ships on the high seas is thus an implicit requirement. The fundamental flight mode is loiter at at least modest altitudes, although significant range, dash speed, and altitude are needed for some applications.
Conventional takeoff and landing (CTOL) aircraft tend to look like small airplanes and readily provide flight at low power loading (gross weight/installed horsepower) and hence loiter well and cruise well, although at relatively low speeds. They can readily be catapulted for takeoff with good fuel loads but require several hundred feet for landing and still more if loiter is compromised to achieve more speed. There are also specialized recovery techniques such as: parachutes, airborne nets, ship-based nets, and arresting hooks and wires, none of which fully address the complete UAV requirement. CTOL configurations are, however, inexpensive to develop and acquire.
Helicopters can provide good loiter and reasonable altitude capability and can easily be launched. They also capitalize on both manned and drone recovery operational experience on ships. Helicopters inherently have limited speed and range capability, however. They are mechanically very complex compared to CTOL configurations and thus much more expensive.
Highly loaded powered lifting discs (turbo jets or turbo fans) have been flown with the axis nearly vertical and the exhaust pointed downward. In some configurations several turbines have been ganged together and suggest "unidentified flying objects" (UFOs). These configurations tend to be very light, compact, and simple. If one calculates the power loading, it is found that the values are extremely high, hence fuel consumption is also extremely high. These configurations are also very inefficient in cruise flight, hence, both loiter and cruise are very poor compared to the previously discussed alternatives. The exhaust velocities are well above hurricane levels, hence, operational sites require special preparation or must be carefully selected. Since the induced velocities at low flight speeds are high compared to wind gusts, however, gust sensitivity is low.
Tilt rotor configurations couple the low speed advantages of the helicopter with the airplane, and thus provide efficient hover, loiter, cruise, and altitude performance. They are more complex and hence more expensive than helicopters and are dimensionally very large for a given weight. They also suffer from gust sensitivity in low speed like the helicopter because their induced velocities are not much larger than sudden wind changes under gusty conditions.
Tilt turbine configurations tilt powered discs in an airplane type configuration and thus provide both hover and good high speed flight. Gust sensitivity is low, but exhaust velocities are high with the attendant site problems. The British Harrier is the only jet V/STOL to date which has operated extensively from ships. It mounts a simple specific one-of-a-kind turbine engine occupying all the fuselage volume at the aircraft center of gravity.